This invention relates generally to a method and apparatus for detecting a frequency synchronization signal. More particularly, this invention relates to a method and apparatus for detecting a frequency synchronization signal transmitted from a transmitter and received by a receiver in a communication system.
In any communication system, it is important for a receiver to be synchronized with a transmitter so that messages can be successfully exchanged between the transmitter and the receiver. In a radio communication system, in particular, it is important that a receiver be tuned to the frequency of the transmitter for optimal reception.
In a typical radio communication system, remote stations communicate with one or more base stations via a radio air interface. Various approaches have been employed to prevent transmissions between the various base stations and remote stations from interfering with each other.
In some radio communication systems, neighboring base stations are each assigned a different carrier frequency with which to communicate with remote stations so that transmissions from one base station do not interfere with transmissions from a neighboring base station. In addition to such a Frequency Division Multiple Access (FDMA) technique, Time Division Multiple Access (TDMA) has been employed. In systems using TDMA, a base station may allocate a particular time slot or slots within a frame on a carrier to each remote station. Some remote stations can use the same carrier frequency but different time slots to communicate with the base station.
In other radio communication systems, the Code Division Multiple Access (CDMA) method has been employed. According to the CDMA method, each remote station is assigned a particular digital code word(s) that is orthogonal to code words assigned to other stations. Neighboring base stations can exchange messages with remote stations using the same frequency but different digital orthogonal code words to indicate which remote station the messages are designated for.
Whether a radio communication system employs FDMA, TDMA, CDMA, a combination of these approaches, or some other approach, it is important for a remote station to be time and frequency synchronized to the base station serving the area from which it desires to communicate. In other words, the local frequency reference of the remote station must be tuned to the carrier frequency of the base station, and the local time reference of the remote station must be synchronized to the time reference of the base station. A periodic synchronization signal is typically transmitted from the base station to the remote station for this purpose.
In a system complying with the European Global System for Mobile Communication (GSM) standard, information is transmitted from the base station to a remote station by modulating the carrier of the base station with, e.g., a Normal Burst (NB) of data. To synchronize the mobile station to the base station, the carrier of the base station is also modulated from time to time with a Frequency Correction Burst (FCB) and a Synchronization Burst (SB) to form a frequency synchronization signal.
The carrier of the base station is typically modulated with the FCB using Gaussian Minimum Shift Keying (GMSK). In a GSM system, a FCB is a sequence of 148 symbols, each symbol a zero, that transforms into a pure sinusoidal signal after modulation. The frequency of the resulting frequency synchronization signal is thus equal to 1/4+L T Hz, where T represents a symbol duration. T is typically 48/13+L microseconds (.mu.s), so that the frequency synchronization signal has a frequency of approximately 67.7 KHz. The FCB is repeated every tenth frame for the first four times, and then for the fifth time, the FCB is repeated on the eleventh frame. This frame sequence is then repeated indefinitely, to maintain synchronization between the remote station and the base station.
From the information in the FCB, the remote station is able to roughly synchronize itself with the time slot(s) allocated to it. This rough time synchronization is then sufficient to locate the SB, which is typically located eight bursts after the FCB, and to decode the information it carries. The information obtained by decoding the SB is then used to finely tune the local frequency reference of the remote station to the carrier frequency of the base station and to adjust the remote station's local time reference to the time slot(s) allocated to it by the base station.
In systems employing CDMA, each base station transmits a frequency synchronization signal in the form of, for example, a pilot sequence on each of the frequencies assigned to that particular base station as well as possibly on some or all of the frequencies that are not assigned to that particular base station. If the frequency has been assigned to the base station, the corresponding pilot sequence may be transmitted with slightly more power than the other frequencies used by the base station. Each remote station receiving the carrier modulated by the pilot sequence demodulates the signal. As a result, each remote station can receive signals designated for it and simultaneously measure the signal strengths of neighboring base stations using different pilots or carriers. This information is used by the remote station to determine which received pilot sequence has the strongest signal strength, and the local frequency reference of the remote station is adjusted to the appropriate carrier frequency, accordingly.
Any frequency difference between the local frequency reference of the remote station and the carrier frequency of the base station is readily detected in the demodulated frequency synchronization signal. For example, in systems complying with the GSM standard, the difference between the frequency of the modulated frequency synchronization signal, which is known to be 67.7 KHz, and the frequency of the received frequency synchronization signal, demodulated to the baseband, is a direct measure of the error in the local frequency reference of the remote station. In systems employing CDMA, the difference between the known frequency of the strongest transmitted pilot sequence and the frequency of the demodulated pilot sequence is used by the remote station as a measure of the error in the local frequency reference of the remote station.
In order to synchronize a remote station to a base station, it is therefore important to accurately detect the frequency synchronization signal transmitted from the base station. Many techniques have been proposed for detecting the frequency synchronization signal, one of which is disclosed in C. Fjelner and E. Sward, Implementation of Digital Frequency Correction Burst Detector with Field Programmable Gate Arrays (1994) (Master thesis, Lund University). This technique is designed for detection of an FCB modulated signal in a GSM system. According to this technique, the FCB modulated signal can be detected by measuring the phase of the received signal from the base station as a function of time and differentiating this phase. Since the FCB transforms into a sinusoidal signal after modulation, the phase of the FCB modulated signal transmitted from the base station increases linearly as a function of time. Therefore, differentiation of the phase of a received signal corresponding to the FCB results in a constant. Unlike the phase of a signal modulated with an FCB, the phase of a signal modulated with, e.g., an NB does not increase linearly as a function of time. Therefore, differentiation of a phase of a received signal which does not correspond to the FCB does not result in a constant. By differentiating the phase of a received signal and examining the results, it can thus be determined whether the received signal corresponds to an FCB.
FIG. 1 shows an implementation of this technique. As shown in FIG. 1, the phase of a received signal y(n) at time n is measured in a Phase Measuring Circuit 10 which outputs the phase .phi..sub.y (n) of the received signal as a function of time n. The phase .phi..sub.y (n) is differentiated and unwrapped in a Differentiator/Unwrapper 20. The phase can vary between successive samples by an amount between 0 and 2.pi.. To keep the differentiated phase bounded in the interval (-.pi., .pi.) and thus simplify the detection process, the phase .phi..sub.y (n) can be shifted, i.e., "unwrapped", prior to differentiation to eliminate the 2.pi. jumps. Alternately, the phase can be unwrapped after differentiation, e.g., by shifting the differentiated phase by 2.pi. or -2.pi. when the differentiated phase is larger or smaller than .pi. or -.pi., respectively.
The differentiated and unwrapped phase .DELTA..phi..sub.i is filtered in a Low Pass (LP) Filter 30. The output (.DELTA..phi..sub.i).sub.LP of the LP Filter 30 experiences a dip or jump for an input differentiated phase corresponding to an FCB. Thus, an FCB can be detected simply by detecting a dip or jump in the output (.DELTA..phi..sub.i).sub.LP.
A modified version of the conventional differentiation-based detection technique is also illustrated in FIG. 1. According to this modified technique, a Frequency Selective Filter 5, indicated in FIG. 1 by dashed lines, can be used to filter out corrupting noise v(n) outside the frequency region surrounding the carrier frequency of the FCB. This modified approach improves the signal-to-noise ratio (SNR) of the detection system.
Though simple to implement, the modified and unmodified conventional differentiation-based techniques of FIG. 1 may falsely classify signals as FCBs which are not FCBs. The linearly increasing phase of an FCB results from the fact that the FCB is typically obtained by modulating the carrier with a sequence of zeros (0, 0, 0, . . . ). A linear phase can also be obtained for signals which do not correspond to an FCB, e.g., a sequence of ones (1, 1, 1, . . . ) or alternate bits (1, 0, 1, 0, . . . ). Using the approach illustrated in FIG. 1, these sequences result in the same LP Filter output as an FCB. However, it is unlikely that 148 bits in a row are 111 . . . or 1010 . . . Thus, when a 148-bit-long sequence of all zeroes, which corresponds to a linear phase, is obtained, this is an indication that the signal detected is an FCB.
There is thus a need for a technique for accurately detecting a frequency synchronization signal which overcomes the drawbacks noted above.